Pulse-echo ultrasonic imaging method for eliminating sample thickness variation effects

Abstract

A pulse-echo, immersion method for ultrasonic evaluation of a material which accounts for and eliminates nonlevelness in the equipment set-up and sample thickness variation effects employs a single transducer and automatic scanning and digital imaging to obtain an image of a property of the material, such as pore fraction. The nonlevelness and thickness variation effects are accounted for by pre-scan adjustments of the time window to insure that the echoes received at each scan point are gated in the center of the window. This information is input into the scan file so that, during the automatic scanning for the material evaluation, each received echo is centered in its time window. A cross-correlation function calculates the velocity at each scan point, which is then proportionalized to a color or grey scale and displayed on a video screen.

What is claimed is: 
     
       1. A pulse-echo, immersion method for ultrasonic evaluation of a material, employing automatic scanning and digital imaging to obtain an image of a property of said material, wherein said material is held in a holding apparatus which is positioned in an immersion liquid over an acoustic reflector, said reflector having an acoustic impedance which is greater than that of said liquid, and wherein nonlevelness in said holding apparatus and material thickness are accounted for and eliminated, said method comprising: (i) ultrasonically scanning said material at a plurality of scan points and receiving the first and second echoes, each of which is a complete waveform, reflected off the back surface of said material and the first echo reflected off the front surface of said reflector both with and without the presence of said material;   (ii) adjusting the time delay for each said received echo from each said scan point during said scanning in (i) above, gating each said received echo so that it is centered within its respective time window;   (iii) automatically scanning said material at said scan points to receive said first and second back surface echoes and said two front surface echoes using the information obtained in (ii) above, so that each echo received from each scan point during said automatic scanning is centered within its time window;   (iv) digitizing each echo received during said automatic scanning and determining the time delay between said first two successive sample back surface echoes, 2τ, and the time delay, Δt, between the two different reflector front surface echoes received at each scan point during said automatic scanning and calculating the wave velocity, using a cross correlation function, at each said scan point from ##EQU3##  where c is the speed of the ultrasonic wave transmitted in said liquid, and   (v) scaling the velocity values obtained in (iv) to corresponding proportional color or grey scale values and displaying the resulting image.   
     
     
       2. A method according to claim 1 wherein a single transducer is used. 
     
     
       3. A method according to claim 2 wherein said transducer is a high frequency transducer which emits a frequency between 1-100 MHz. 
     
     
       4. A pulse-echo, immersion method for ultrasonic evaluation of a material employing a single transducer, automatic scanning and digital imaging to obtain an image of a property of said material, wherein said material has a uniform thickness variation and is positioned in an immersion liquid between said transducer and an accoustic reflector, said method comprising: (I) accounting for and eliminating nonlevelness in the set-up and said material thickness variation by; (a) performing a preliminary scan along both the x-and y-directions of the material to provide slant correction factors which are input into a computer to account for said nonlevelness and thickness variation during the subsequent automatic scanning for said material evaluation in (ii) below;   (b) adjusting the time delay during said preliminary scan for any received echoes which are not centered in the scan time window, so that each received echo is centered in its time window;     (ii) automatically scanning said material at a plurality of scan points in both the x- and y-directions to receive the first and second back surface echoes and front surface echoes with and without the presence of said material between said transducer and reflector using the information obtained in (i) above, so that each echo received from each scan point during said automatic scanning is centered within its time window;   (iii) digitizing each echo received during said automatic scanning and determining the time delay between said first two successive sample back surface echoes, 2τ, and the time delay, Δt, between the two different reflector front surface echoes received at each scan point during said automatic scanning and calculating the wave velocity at each said scan point from ##EQU4##  where c is the speed of the ultrasonic wave transmitted in said liquid, and   (iv) scaling the velocity values obtained in (iii) to corresponding proportional color or grey scale values and displaying the resulting image.   
     
     
       5. A method according to claim 4 wherein said back and front surface echoes received from the first and last scan points in both the x- and y-directions during said preliminary scan determine said time base adjustments needed for each echo to be centered within the time frame for it. 
     
     
       6. A method according to claim 5 wherein the location of the time window during said automatic scanning for said material evaluation is automatically adjusted via computer control by using the formula:   W.sub.DT =T.sub.I +[(X.sub.SC)(X.sub.SN)(X.sub.SI)+(Y.sub.SC)(Y.sub.SN)(YSI)]     wherein W DT  is the correct delay time window at a particular scan location, T I  is the time delay at the the initial scan location, X SC  and Y SC  are the x- and y-direction slant correction factors, X SN  and Y SN  are the scan point numbers in the x- and y-directions, and X SI  and Y SI  are the x- and y-direction scan increments.   
     
     
       7. A method according to claim 6 wherein two scans are automatically made to obtain said first two back surface echoes and said two different reflector front surface echoes. 
     
     
       8. A method according to claim 7 wherein said first two back surface echoes and said reflector echo with said material present are made in said first scan. 
     
     
       9. A method according to claim 6 wherein three scans are automatically made to obtain said first two back surface echoes and said two different reflector front surface echoes. 
     
     
       10. A method according to claim 9 wherein said first two back surface echoes are received in one scan, wherein said reflector echo with said material present is made in another scan, and wherein and said reflector echo without said material present is received in yet another scan. 
     
     
       11. A method according to claim 9 wherein said first back surface echo is received in said first scan, wherein said second back surface echo is received in said second scan, wherein said reflector echo with said material present is made in said third scan, and wherein said reflector echo without said material present is received in said fourth scan. 
     
     
       12. A method according to claim 6 wherein four scans are automatically made to obtain said first two back surface echoes and said two different reflector front surface echoes. 
     
     
       13. An ultrasonic, pulse-echo, immersion method employing automatic scanning and digital imaging to obtain an image of a microstructural property of a material positioned in an immersion liquid between a transducer and an acoustic reflector, said method comprising: (i) automatically scanning said material at least three times at a plurality of scan points in both the x- and y-directions to receive the first and second back surface echoes and front surface echoes with and without the presence of said material between said transducer and reflector, each of said echoes received being a complete waveform and gated within a time window;   (ii) digitizing each echo received during said automatic scanning and determining the time delay between said first two successive sample back surface echoes, 2τ and the time delay, Δt, between the two different reflector front surface echoes received at each scan point during said automatic scanning and calculating the wave velocity at each said scan point from ##EQU5## where c is the speed of the ultrasonic wave transmitted in said liquid, and   (iii) scaling the velocity values obtained in (ii) to corresponding proportional color or grey scale values and displaying the resulting image.   
     
     
       14. A method according to claim 13 wherein four scans are automatically made to obtain said first two back surface echoes and said two different reflector front surface echoes. 
     
     
       15. A method according to claim 14 wherein said first back surface echo is received in said first scan, wherein said second back surface echo is received in said second scan, wherein said reflector echo with said material present is made in said third scan, and wherein and said reflector echo without said material present is received in said fourth scan.